Rigging and Bone Weighting: Ignore Y Rotation (Twisting), Only Follow X & Z Rotation (Bending)
The simplest way do this is to add another bone that is a copy of the bone with twist (e.g create a forearm-notwist bone from the forearm bone). Make sure it is the same size with the same parent, location and orientation as the bone it copies. 

Then add a damped track constrain to your no-twist bone, pointing to your original (e.g. forearm) bone, and change the head/tail slider to 1 (point at the tail. 

If you need to you can also add copy location and copy scale constraint to the new bone, depending on what other transforms may be applied to the original.